$BASH_SOURCE is the call stack of scripts with the top of the stack being the currently executing sourced script, sourcing another script from the current one will push it on top of the stack. To get an idea of how this works you can set up a directory tree to provide a variety of ways to call the same script:

cd /tmp  
mkdir -p 1/2/3/4/5/6/7/8/9/0  
vim  1/2/3/4/5/6/7/8/9/0/bash_source.sh  

Inside the script we'll print out the contents of the $BASH_SOURCE array for the current invocation and then if the array isn't too long already we'll recursively call the script again from further down the directory tree:

#!/bin/bash

for I in $(seq 0 $(expr ${#BASH_SOURCE[@]} - 1) ); do  
        echo BASH_SOURCE\[$I\] ${BASH_SOURCE[$I]}
done  
echo

if [[ ${#BASH_SOURCE[@]} -le 10 ]]; then  
        cd [0-9] # head up a directory
        . ${BASH_SOURCE[0]#[0-9]/} # call top of the stack, minus leading directory
fi  

And this is what you end up with. Initially the $BASH_SOURCE array is just the initial invocation, but with further recursive sourcing it continues to grow:

./1/2/3/4/5/6/7/8/9/0/bash_source.sh
BASH_SOURCE[0] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[1] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[1] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[2] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[1] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[2] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[3] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[1] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[2] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[3] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[4] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 6/7/8/9/0/bash_source.sh  
BASH_SOURCE[1] 5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[2] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[3] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[4] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[5] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 7/8/9/0/bash_source.sh  
BASH_SOURCE[1] 6/7/8/9/0/bash_source.sh  
BASH_SOURCE[2] 5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[3] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[4] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[5] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[6] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 8/9/0/bash_source.sh  
BASH_SOURCE[1] 7/8/9/0/bash_source.sh  
BASH_SOURCE[2] 6/7/8/9/0/bash_source.sh  
BASH_SOURCE[3] 5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[4] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[5] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[6] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[7] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 9/0/bash_source.sh  
BASH_SOURCE[1] 8/9/0/bash_source.sh  
BASH_SOURCE[2] 7/8/9/0/bash_source.sh  
BASH_SOURCE[3] 6/7/8/9/0/bash_source.sh  
BASH_SOURCE[4] 5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[5] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[6] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[7] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[8] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] 0/bash_source.sh  
BASH_SOURCE[1] 9/0/bash_source.sh  
BASH_SOURCE[2] 8/9/0/bash_source.sh  
BASH_SOURCE[3] 7/8/9/0/bash_source.sh  
BASH_SOURCE[4] 6/7/8/9/0/bash_source.sh  
BASH_SOURCE[5] 5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[6] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[7] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[8] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[9] 1/2/3/4/5/6/7/8/9/0/bash_source.sh

BASH_SOURCE[0] bash_source.sh  
BASH_SOURCE[1] 0/bash_source.sh  
BASH_SOURCE[2] 9/0/bash_source.sh  
BASH_SOURCE[3] 8/9/0/bash_source.sh  
BASH_SOURCE[4] 7/8/9/0/bash_source.sh  
BASH_SOURCE[5] 6/7/8/9/0/bash_source.sh  
BASH_SOURCE[6] 5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[7] 4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[8] 3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[9] 2/3/4/5/6/7/8/9/0/bash_source.sh  
BASH_SOURCE[10] 1/2/3/4/5/6/7/8/9/0/bash_source.sh